This is a cross-sectional and longitudinal study of the natural history and pathogenesis of MAC disease in late-stage HIV infected subjects and the impact of highly active anti-retrviral treatment (HAART) on immune functions relevant for MAC and other opportunistic infections. Up to 100 evaluable subjects will be enrolled. The primary objective is to determine if tissue infection with MAC occurs prior to sustained bacteremia by obtaining bone marrow biopsies and blood for quantitative cultures from subjects at high risk for DMAC.